1. Field of the Invention
This invention relates to video post-production of material acquired on film, and more particularly to the use of high-definition video equipment for the post-production of material acquired on movie film.
2. Description of the Prior Art
60 Hz, that is 60 fields per second, 2:1 interlace format high-definition video equipment is now well known and in widespread use. Despite this, movie film remains a very good way of acquiring material, even where the material is subsequently to be distributed in the form of high-definition video. This is not only because of the favourable rendition of material on film, but also due to the relative portability of film cameras compared with the current generation of high-definition video cameras, and also the ability of film cameras to produce good slow motion by acquisition at high frame rates.
Nevertheless, it is desirable to enable material acquired on film, normally at 24 Hz, but possible at 25 Hz or 30 Hz, to be post-produced using 60 Hz 2:1 interlace format high-definition video equipment, even in cases where the material may subsequently be transferred back to film for distribution. This is because there are many processes and effects which can readily be carried out on a video signal, for example, digital multi-effects, the integration into the film material of live video material, manipulation of the image, or even merely the simple addition of captions or titles, which are very expensive to do optically. Moreover, it may be desired to apply such processes and effects to material held in film archives.
FIG. 1 shows in block form a possible system for video post-production of material which has been acquired on film and is to be distributed on film or video. 24 Hz 1:1 format film material 1 is read by a telecine 2 to derive a 60 Hz (60 fields per second) 2:1 interlace format video signal. This may be done, for example, using 3 2 3 2 pull down, referred to in more detail below, and as currently used, for example in Japan and USA, for converting film material to a video signal. The video signal is supplied to a post-production unit 3 from which the modified video signal is supplied to an electron beam recorder system 4 for recording on film material 5 in a 24 Hz 1:1 format, for example using a drop field technique. The modified video signal may also be supplied to a video recorder 6 for recording in 60 Hz 2:1 interlace format.
It might be considered that the initial transfer from film to video could be done directly, in the sense that each frame of the film became a frame of video. Then, following post-production, each frame of video could be transferred directly back to a film frame. However, if this method were used then the video signal in the post-production unit 3 could not be viewed at real time speeds, because all action would be speeded up by a factor of 30/24. This would make it difficult to evaluate the aesthetic aspects of the program material during the post-production process. Moreover, the modified video signal could not be directly transferred to the video recorder 6 for distribution in video form. This possibility is not, therefore, a practical option.
The alternative is as indicated above in connection with FIG. 1, in which there is field repetition in the transfer from film to video. FIG. 2 illustrates the 3 2 3 2 pull down system, and shows the correspondence between film frames and video fields. This is shown for 24 Hz 1:1 format film transfer to 60 Hz (30 frames per second) 2:1 interlace format video. It will be seen that film frame 1 is used to form video fields 1, 2 and 3, video field 3 being a so-called phantom field because it is a repeat of video field 1. Film frame 2 is then used to form video fields 4 and 5. This gives an odd-even-odd-even-odd sequence of five video fields derived from the two film frames. To maintain the required odd/even alternation in the video fields, the 5-field sequence obtained then has to be repeated starting with an even field. Thus, film frame 3 is used to form video fields 6, 7 and 8, and film frame 4 is used to form video fields 9 and 10. In this case, video field 8 is the phantom field because it is a repeat of video field 6. Repetition of this 10-field sequence gives the required odd/even alternation.
This method produces correct speed of action during video post-production, although motion characteristics will be slightly jerky due to the phantom fields, that is the repeated video fields in each 5-field sequence. Although, as mentioned above, this 3 2 3 2 pull down method is commonly used for film to video transfer in USA and Japan, the motion portrayal is not strictly correct, although it is acceptable to viewers.
Video post-production equipment currently available is generally intended to operate on a video signal which is interlaced, that is a signal in which the two fields which compose a frame are equally separated in time from adjacent fields. Any post-production equipment that produces motion, such as special effects generators, or equipment which generates wipes or moving captions, therefore operates in such a way as to produce a regular motion across video fields. An example of this is shown for a switcher wipe in FIG. 3, where a wiping edge moves regularly across the picture from side to side. Fields 1 to 6 shown on the left correspond to fields 1 to 6 from FIG. 2; field 3 being one of the phantom fields in the 10-field sequence. The arrows in the centre of the figure indicate the fields selected for transfer to film, the image on which is indicated on the right of the figure. The areas within the concentric circles indicate a magnified view of the line structure derived from the video signal. The hatched areas in the images show the smear regions resulting from the composition into the same frame of video fields in which the motion of the wipe is different. It will also be noted that in particular due to dropping the phantom field, there is a sharp jump in the motion due to the dropped field. What has happened, is that there is now an incorrect temporal sequence as a result of dropping the phantom field, and so the motion is smeared.